2 x Hydrogen & 1 x Oxygen

[schmack]

Hi all, it's been a while since i've made a decent post on SDC , but i was asked by a young person the other day "Why doesn't water burn?" It's made out of two of the most explosive elements ie: Hydrgen and Oxygen, yet it's non flamable. And i didn't know the answer so i thought i'd pop in to SDC and ask you guys.

 

[kelvinzero]

In a way water is not explosive exactly because hydrogen and oxygen are.
Hydrogen and oxygen release a lot of energy when they combine to form water.
This means you would have to put a lot of energy into water to break it back into hydrogen and oxygen.
If you have to put energy in to something to make it do anything, it is stable rather than explosive.

To put it another way, it is about potential energy. Instead of hydrogen and oxygen, consider a boulder and a cliff.

• A boulder at a top of a cliff is an unstable situation. It is poised to do something very violent. Possibly to a cartoon coyote.
• However, for the same reasons, the boulder at the bottom of the cliff is a very stable situation. All its destructive energy is spent. It cannot not do anything violent until the coyote does a lot of work to push it up to the top of the cliff again.

 

[schmack]

Thanks Kelvinzero! I think i'll even use the "kyote" analogy when i explain it to the young person!!

Cheers!

 

[origin]

kelvinzero is correct, but curiously enough you can burn water - you just need to put alot of energy into it. If you could manage to sprayed water onto a 2000 C object the water will disassociate into hydrogen and oxygen and all that hydrogen and oxygen around the heat source will recombine, that is burn.

 

[SteveCNC]

It almost strikes me that water is the end result of hydrogen burning , most fire is the result of oxidation is it not ? So water is already burnt hydrogen.

 

[LKD]

This reminds me of: http://www.youtube.com/watch?v=hw1xhRt9GvU

Nice explanation. Very simple and understandable kelvinzero.

 

[Ishimura_]

I'd say it has to do with emergent properties too. Water is more than the sum of its parts and therefore is an entirely new entity. It's the same logic for NaCl - Sodium is explosive in water; Chlorine is a poisonus gas; combine the two and you have salt, something that is required for our life.

 

[origin]

I'd say it has to do with emergent properties too. Water is more than the sum of its parts and therefore is an entirely new entity. It's the same logic for NaCl - Sodium is explosive in water; Chlorine is a poisonus gas; combine the two and you have salt, something that is required for our life.

I know this probably sounds picky but...

It is not that the sodium and the chlorine combine. It is that the chlorine becomes a chloride (the atom gains an electron) and the sodium losses an electron and then they combine. The differeces in the charges are there prior to the formation of salt, the charge difference is what makes the formation of NaCl. In sea water there is no NaCl (generally), there are Na+ and Cl- floating around as ions, if you evaproate the water off you will get NaCl though.

 

[Ishimura_]

I know this probably sounds picky but...

You're correct, it does sound picky

The point being that the new entity, NaCl, has properties that differ from the properties of its constituent atoms. The same with water and although Hydrogen and Oxygen have atomic properties different from that of water, it is due to the fact that water, although composed of H and O, is a new entity in itself, that has properties different from its constiutent atoms. The polar covalent bond (or hydrogen bond) takes a lot of energy to break compared to a non-polar covalent bond found in hydrocarbons.

 

[origin]

I know this probably sounds picky but...

You're correct, it does sound picky

The point being that the new entity, NaCl, has properties that differ from the properties of its constituent atoms. The same with water and although Hydrogen and Oxygen have atomic properties different from that of water, it is due to the fact that water, although composed of H and O, is a new entity in itself, that has properties different from its constiutent atoms. The polar covalent bond (or hydrogen bond) takes a lot of energy to break compared to a non-polar covalent bond found in hydrocarbons.

I was just pointing out that water is, as someone noted oxidized hydrogen. There is a very exothermic reaction when hydrogen combines with oxygen to fomr a new compound. That is not what is happening in the formation of NaCl. That is because NaCl is an ionic compound so it is NOT composed of Na and Cl it IS composed of NA+ and Cl-. So if you eat a grain of salt or drink water that has salt's constituents, Na+ and Cl- you will get the same affect. If however you were to come in contact with any significat amount of the non-ionized forms of Na+ and Cl- it would ruin your whole day.

It is a rather subtle difference but an important distinction - nit picky if you like.

 

[MeteorWayne]

Just to point out that the Main Engines of the Shuttle "burn" hydrogen and oxygen for the thrust that accelerates it after the Solid Rocket Boosters burn out for more than 6 minutes to get it into orbit.

In fact they burn 65,000 gallons of liquid hydrogen and oxygen per minute!

 

[Kessy]

It depends on what exactly you mean by "burn." In everyday usage, something burns when a material like organic compounds, hydrogen, or something similar reacts with oxygen. For these materials, the chemical reaction is quite exothermic, meaning it releases a lot of energy into the environment in the form of heat. This heat is enough to turn the gases right around the reaction into plasma - a flame.

If you want a reaction with oxygen, then no, water won't burn. As was stated earlier, water is the product of hydrogen burning, so it's kind of like ash.

If you just want an exothermic reaction, then you can burn water, you just need something other then oxygen for it to react with. Alkali metals like sodium work well.

If you want a plasma like a flame. you just need to heat the water a lot by whatever means. Most material will turn into a plasma if you make them hot enough.

 

[drwayne]

ClF3 will react quite well with water - or badly, depending on your point of view of the reaction.

 

[kelvinzero]

kelvinzero is correct, but curiously enough you can burn water - you just need to put alot of energy into it. If you could manage to sprayed water onto a 2000 C object the water will disassociate into hydrogen and oxygen and all that hydrogen and oxygen around the heat source will recombine, that is burn.

Yes, I guess that the bottom of one cliff might be the top of another cliff.
(edit: oops I missread this: I was thinking in terms that even water can still react violently with other chemicals. The example above is still the same cliff )

There was a recent article about an effective rocket fuel using ice and aluminium powder! Hard to believe but I guess if the aluminium wants to bond to the oxygen even more than the hydrogen..

http://www.popsci.com/military-aviation ... -water-ice

 

[origin]

kelvinzero is correct, but curiously enough you can burn water - you just need to put alot of energy into it. If you could manage to sprayed water onto a 2000 C object the water will disassociate into hydrogen and oxygen and all that hydrogen and oxygen around the heat source will recombine, that is burn.

Yes, I guess that the bottom of one cliff might be the top of another cliff.

I think your analogy of the cliff was spot on. To extend the analogy way to far - the 2000C energy source effectively just takes the boulder back up to the top of the cliff. :geek:

 

[drwayne]

ClF3 will react quite well with water - or badly, depending on your point of view of the reaction.

And of course whether you have a good pair of running shoes on.

Sorry - a little bit of that "Ignition" humor leaking though

 

[origin]

ClF3 will react quite well with water - or badly, depending on your point of view of the reaction.

And of course whether you have a good pair of running shoes on.

Sorry - a little bit of that "Ignition" humor leaking though

I hadn't heard of the this lovely compound before. It violently reacts with water to form chlorine and hydrofloric acid? Oh, gee I wish I could work with that compound - not. YUCK! :shock: I work with TiCl4 and that seems positively benign in comparison.... I do not like HF - even a little.[shudder]

 

[drwayne]

I had to use HF to etch and clean quartz sample tubes - ack

 

[killium]

Burning is when something binds with Oxygen. So theoretically, H2O2 (Hydrogen peroxide) IS burnt water!

 

[theridane]

Burning is when something binds with Oxygen. So theoretically, H2O2 (Hydrogen peroxide) IS burnt water!

Combustion (burning) is an exothermic reaction between a fuel and an oxidizer. By definition.

In order to manufacture H[sub]2[/sub]O[sub]2[/sub] you need to add energy, it's heavily endothermic. And it's not made by oxidizing water. So.. nah.

Also, products of burning have axiomatically lower energy than the sum of their constituents - because they released some of it during the exothermic burn. Hydrogen peroxide on the other hand eagerly (and aggresively) decomposes into water and oxygen, while releasing energy (that's why it's used as rocket propellant). If it released energy during the manufacture and during the decomposition, you'd have some serious first law violation charges on your record. So again, it's not burnt water, not theoretically, not in any other way.

Burning is when something binds with Oxygen

We have other oxidisers, like fluorine. It's even more powerful than oxygen - there are rocket tri-propellants that utilize F as an oxidiser and have extremely high specific impulses, way higher than LOX/LH[sub]2[/sub] compositions. And I'm pretty sure they call that "burning" too.

 

[drwayne]

Actually there has been some semi-serious work with combinations of Flourine and LOX,
giving it the name FLOX. We have discussed it here, as well as the general tendency of
Flourine to be hypergolic with a lot of the things one would find on this planet.